Aerospace fiber optic applications present a difficult design challenge relative to other fiber optic applications due to the large number of bulkhead disconnects that may be required, and the relatively high attenuation that occurs at each of these bulkhead connectors. The high attenuation may be the result of the unique environment that the connectors operate in, especially with respect to vibration and electrical signal contamination. As a result, high-speed fiber optic networks using convention connection apparatus and methods may not be possible in some aerospace applications.
One conventional design technique that can improve the attenuation problem is to use an Avalanche Photodiode (APD) at connections of the optical system instead of a more conventional photodetector. The APD may provide internal gain that can result in several dB of signal-to-noise (S/N) improvement. This may be enough S/N improvement to make possible the use of several additional connectors. The additional S/N may also make possible the use of other lossy components, such as optical switches.
A difficulty of using the APD may arise, however, because the APD's characteristics exhibit variation with temperature. With limited temperature variation (e.g. within an office building), the temperature effects can be compensated by measuring the temperature and adjusting the high-voltage bias (and therefore the gain) on the APD to compensate. In some aerospace environments, however, the temperature of the APD can range from −40° C. to +100° C. Temperature compensation over such a wide range is generally quite difficult to achieve. Furthermore, there is a potential problem in such applications because the APD may be operated within a few volts of a breakdown voltage, and that breakdown voltage typically changes with temperature (part of the APD characteristics that change with temperature). APD breakdown, while not catastrophic, typically renders the device useless for communications until it is brought back (by reducing the bias voltage) into normal operation. Thus, a link relying on an APD for receiver detection will drop out when such breakdown occurs. Therefore, there is an unmet need for fiber optic systems that provide improved S/N performance in relatively demanding environments, particularly environments characterized by extreme temperatures, vibration and electrical signal contamination of the type which may exist in some aerospace environments.